Electromechanical rotary actuators have been in existence for decades. They are used in a variety of industrial and consumer applications, but they are particularly useful in the field of optical scanning, where an optical element is attached to an actuator output shaft, which is then rotated back and forth in an oscillating manner.
For example, it is common to attach a mirror to the output shaft of a rotary actuator in order to create an optical scanning system. In this application, the actuator/mirror combination can redirect a beam of light through a range of angles, or redirect the field of view of a camera so that it can observe a variety of targets.
Other optical elements can be attached to the output shaft as well. For example, a prism or an optical filter can be attached to the shaft and the rotation of the actuator shaft can vary the angle of the prism or filter. If a dielectric filter is used, changing the filter's angle-of-incidence will shift the band-pass wavelength characteristics higher or lower, thus allowing the optical system to be tuned to a particular wavelength. Alternatively, the prism or filter can be rotated completely into and out of the beam path, thus allowing selective filtering of the beam.
Typical electromechanical rotary actuators used in the field of optical scanning are generally made from some combination of magnet, steel and coils of insulated “magnet” wire. These elements have been arranged in a variety of ways, but for the past twenty years, the most popular arrangement has been to use a simple two-pole rotor magnet, and a “toothless” stator design.
The rotor within these actuators is typically a solid, cylindrical magnet made from high grade Neodymium Iron Boron which is diametral magnetized, and onto which two shafts are attached. One shaft portion may be attached to a mirror, and another shaft portion operable with a position sensor. The shaft is typically supported by ball bearings. By way of example, dimensions for this disclosure may comprise a rotor magnet having a diameter of 0.12 inches (around 3 millimeters) and a length of 1.3 inches (around 33 millimeters).
It will be helpful to review known actuator technology and make reference to known actuators to have the reader better understand the needs satisfied by embodiments of the present invention. While addressing problems in the art in this background section of the disclosure, it will also be helpful to describe developing embodiments generally accomplished through extensive analysis and experimentation. Therefore, all the disclosure included in this background section should not be construed as being a known prior art teaching.
FIG. 1 illustrates a sectional view of the rotor and stator arrangement found in a typical “toothless” optical scanner of the current state of the art. The stator is essentially tubular. For the rotor magnet diameter described above, a typical stator tube may have an outside diameter of 0.5 inches (around 12.7 millimeters), an inside diameter of 0.196 inches (around 5 millimeters), and is typically made from cold rolled steel. Coils of magnet wire are formed and bonded to the inside wall of the stator steel tube, occupying around a 90 degree arc. There is typically around a 0.007 inch gap between the outside wall of the rotor magnet and the inside wall of the coil, thus allowing the magnet to rotate freely. Within FIG. 1, the coil areas are designated “Coil plus” and “Coil minus” to indicate turns going into the page and turns coming out of the page, respectively.
FIG. 2 illustrates magnetic field lines found in a typical “toothless” optical scanner of the current state of the art as illustrated in FIG. 1. It can be seen that the magnetic flux lines must extend (“jump”) across a relatively large gap to reach the stator steel. The coil resides in between the magnet and the stator steel. When the coil is energized, a Lorentz Force is imposed on both the coil and the magnet. Since the coil is typically bonded to the stator and thus, held stationary, all of the force is conveyed to the rotor magnet. Since force is created on opposite sides of the magnet, the force being in the form of torque, the actuator creates torque and thus creates motion.
In this example of an actuator, there are 50 turns of AWG #33 magnet wire used, having a coil resistance (R) of around 2.5 ohms, a coil inductance (L) of around 100 microhenries, and producing a torque constant (KT) of around 38,000 Dyne*Centimeters torque per Amp of electrical current passed through the coil.
The toothless arrangement provides benefits. One benefit is the relatively low coil inductance that results from the fact that the coil does not completely surround a closed steel core. Quite the contrary, the entire inside of the actuator is open, containing only the rotor magnet whose permeability is almost the same as that of air.
However, the toothless structure is not without drawbacks. One primary drawback is the amount of heat generated during fast/wide angular rotor motions. Further, the heat that is generated cannot be removed effectively. Both of these drawbacks stem from the fact that, the coil occupies a relatively small space (cross-sectional area), and that it is bonded to the inside of the stator tube, so that it only has a direct attachment on one side (the outside of the coil).
Referring again to FIG. 1, it can be seen that the left, right, and inside of the coils are essentially not attached to any surfaces. Because of this, heat generated by the coil can only be removed from one surface (the outside). Indeed, heat generated at the inside surface of the coil tends to heat up the rotor magnet, which degrades performance and can risk demagnetizing the rotor magnet if the heat exceeds around 100 degrees C.
In order to generate less heat, a lower coil resistance is needed, and in order to decrease the coil resistance, thicker wire must be used.
If, for example, AWG #29 magnet wire was used instead of AWG #33 magnet wire, and was placed into the same coil area, only around 22 turns could be used, providing a coil resistance (R) of 0.48 ohms and a torque constant (KT) of 16,720 Dyne*Centimeters per amp. The coil resistance is certainly lower (because of the thicker wire), but the torque constant is also lower (because there are fewer turns).
When comparing motor designs, it is useful to use figures of merit. One important figure of merit is referred to as a motor constant (KM), which indicates the amount of heat generated for a given amount of torque produced by the actuator. The KM can be calculated several ways, but the easiest way is: KM=KT/√R.
The KM of the original actuator with 50 turns, whose KT=38,000 and R=2.5 ohms is 24,033 Dyne*Centimeters per square root of watt. Therefore, to generate 24,033 Dyne*Centimeters of torque, the motor will need to dissipate 1 watt of heat. To generate twice this amount of torque, or 48,066 Dyne*Centimeters, the motor will need to dissipate 4 watts of heat. Doubling the torque output requires doubling the electrical current input. Since heat is proportional to current squared, it illustrates that doubling the current creates four times the heat.
Comparing these values to the same actuator with 22 turns of AWG #29, whose KT=16,720 and R=0.48, reveals that the KM is now 24,133 or, roughly the same as it was before.
This demonstrates an important law of moving magnet actuators. The KM is dictated by the area allocated for the coil. It does not matter how many turns of wire occupy the coil area. If the coil area remains the same and is fully filled with turns, then the KM will remain the same.
For this reason, it is tempting to simply increase the coil area, for example, by increasing the outside diameter of the coil (and inside diameter of the stator tube). However, increasing the diameter of the stator tube will increase the magnetic air-gap, across which the magnetic flux must jump.
Another figure of merit used in magnetic design is called the Permeance Coefficient (PC). The Permeance Coefficient indicates the “operating point” of the rotor magnet. For a simple circuit consisting of magnet, air, and high permeability steel, the Permeance Coefficient can be found by dividing the Magnetic Length, by the total magnetic air-gap. For the electromechanical actuator described above, having rotor diameter (magnetic length) of 0.120 and stator inside diameter of 0.196 inches, the magnetic air-gap is 0.196−0.120=0.076 inches. Therefore the Permeance Coefficient is roughly 0.120/0.076=1.6.
FIG. 3 provides a B/H curve of a typical high performance Neodymium Iron Boron Magnet. The X axis represents the coercivity (H) of the magnet. The Y axis represents the flux density (B). The numbers around the outside (starting at 0.1 and ending at 5.0 on this plot) are the Permeance Coefficient, which dictates the “operating point” of the magnet. This plot illustrates that at a Permeance Coefficient of 1.6 (as is the case for a typical actuator used in the current state of the art), the magnet operates at a flux density of 8.7 kilogauss when the temperature is 20 degrees C.
If the inside diameter of the stator tube is increased to 0.24 inches, by way of example, this will provide more than double the area for coil wires, easily allowing more than 22 turns of AWG #29 magnet wire to be used. However, increasing the inside diameter of the stator tube also increases the magnetic air-gap that the magnetic flux must jump across. Because of this, the magnetic field becomes weaker. This is shown in the plot of FIG. 4, indicated by the Permeance Coefficient of 1.0. The weaker magnetic field requires even more coil turns to produce the same torque constant. The lower Permeance Coefficient also creates a risk of demagnetization at elevated temperatures.
Analysis and testing have shown that the KM of a toothless actuator remains roughly the same between a Permeance Coefficient of 1.0 and 2.0, and thus, there is essentially no well-known way to overcome the problem of heat generation within a toothless actuator. Therefore if heat generation is a performance limiting factor, another type of actuator must be sought.
In the past, some companies have tried to overcome the problem of heat generation by using “toothed” (also referred to as slotted) actuators. By way of example, FIG. 5 illustrates a sectional view of one such actuator used in known optical scanners. In a toothed actuator, the coil is not located between the magnet and the stator steel, and instead is wound around a steel core which forms “teeth” around the magnet. Since the coil is no longer located between the magnet and the stator steel the stator teeth can be much closer to the magnet. As a result, the Permeance Coefficient of toothed actuators is much higher than for toothless actuators.
FIG. 6 illustrates the same magnet B/H curves as was shown in FIGS. 3 and 4, but also highlights the resulting flux density when the Permeance Coefficient is 6. Since the magnet is operating at a higher flux density, now only 38 turns of wire is required to generate 38,000 Dyne*Centimeters per amp, given the same rotor magnet described above. And since the coil area is much greater, thicker wire can be used.
Clearly a “toothed” stator arrangement can solve the heat generation problem. However, a new problem emerges which is one of greatly increased electrical inductance (L). For an actuator shown in FIG. 5, by way of example, the inductance is greater than 300 microhenries, which is around three times the inductance of a “toothless” actuator with the same torque constant.
Referring again to the plot of FIG. 6, inductance is increased because of two factors. The first factor is “external fringe lines” which circulate magnetic flux around the coil, but do not interact with the rotor magnet to create torque. A second factor is “tooth-to-tooth” fringe lines which circulate magnetic flux around a gap between teeth and do not create torque, as illustrated with reference to FIG. 7.
To eliminate external fringe lines, the toothed stator could be rearranged, as shown in FIG. 8. In this arrangement, the coils are wound around teeth that are located completely contained inside the stator, essentially forming a series magnetic circuit between the two coils. Indeed this does help to reduce inductance to about 212 microhenries, but this is still more than double that of a toothless actuator that produces the same torque.
To reduce the inductance even further, the tooth-to-tooth fringe must be reduced, and thus the gap between stator teeth must be opened up. For example, if the gap between stator teeth is increased to 0.050, the inductance becomes 180 microhenries. If the gap between stator teeth is increased even further—to 0.070, the inductance becomes 157 microhenries. This is still more than 50% higher than a slotless actuator, but this may be tolerable for certain applications.
However, increasing the gap between stator teeth has negative consequences. The largest being that the actuator will tend to “cog” toward angles away from the center, since the North and South poles of the rotor magnet will strongly orient themselves in the direction of the stator teeth themselves. A small amount of cogging can be tolerated by the servo system located outside the optical scanner, but a large amount of cogging is detrimental to performance and thus, highly undesirable.
For example, with the toothed or slotted actuator described above with reference to FIG. 8, whose gap between teeth is 0.030 inches, the cogging torque is 14,000 Dyne*Centimeters at 20 degrees. When the gap between teeth is increased to 0.036 inches, the cogging torque is 22,000 Dyne*Centimeters at 20 degrees. When the gap between teeth is increased to 0.050 inches, the cogging torque increases to 40,000 Dyne*Centimeters at 20 degrees. When the gap between teeth is increased to 0.070 inches, the cogging torque increases to 85,000 Dyne*Centimeters at 20 degrees. A cogging torque of 14,000 Dyne*Centimeters is tolerable, but higher cogging torques are not.
Since limiting the inductance in a toothed actuator also means increasing the cogging torque, this means that a toothed actuator should not be used if inductance is a performance-limiting factor.
To reiterate, the typical toothless actuator is typically not capable of delivering a high torque constant along with low coil resistance, and a typical toothed actuator is not capable of delivering low coil inductance. Thus, there is clearly a need for an electromechanical rotary actuator that provides high torque constant and low coil resistance along with a low coil inductance.